The isotropic-to-nematic phase transition in hard helices: Theory and simulation

Frezza, Elisa; Ferrarini, Alberta; Kolli, Hima Bindu; Giacometti, Andrea; Cinacchi, Giorgio

doi:10.1063/1.4802005

Cited by 32 publications

(60 citation statements)

References 53 publications

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“…A fundamental open question regards the relationship between macroscopic and microscopic chirality, i.e., between the handedness of the phase and that of the constituent particles [14][15][16][17]. Moreover, it is still to be unambiguously proved that hard chiral interactions alone can give rise to an entropy-stabilized chiral-nematic phase [17,18]. The problem in interpreting experimental data is largely due to the limitations of theory and simulation methods.…”

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confidence: 99%

“…Here we focus on the chiral nematic phase developed by a large class of hard-sphere helices described in Ref. [18] (cf. Fig.…”

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confidence: 99%

“…(b) Hard helix particle modeled as a collection of 15 partially overlapping hard spheres of diameter σ, whose centers of mass are equally spaced on a right-handed helix of contour length L = 10 σ, inner radius r = 0.4 σ and pitch p = 4 σ [18]. The orientation of the helix can be expressed by means of the unit vectorω and the internal azimuthal angle α. chiral-nematic phase [17,18]. The problem in interpreting experimental data is largely due to the limitations of theory and simulation methods.…”

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confidence: 99%

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Density functional theory for chiral nematic liquid crystals

et al. 2014

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Even though chiral nematic phases were the first liquid crystals experimentally observed more than a century ago, the origin of the thermodynamic stability of cholesteric states is still unclear. In this Letter we address the problem by means of a novel density functional theory for the equilibrium pitch of chiral particles. When applied to right-handed hard helices, our theory predicts an entropydriven cholesteric phase, which can be either right-or left-handed, depending not only on the particle shape but also on the thermodynamic state. We explain the origin of the chiral ordering as an interplay between local nematic alignment and excluded-volume differences between left-and right-handed particle pairs.PACS numbers: 64.70.mf, 64.70.pv, 61.30.Cz, 61.30.St Cholesteric phases, also known as chiral nematics, are fascinating examples of liquid crystals. Liquid crystals are phases of matter characterized by a degree of spontaneous breaking of the rotational and translational symmetries that is higher than in the liquid and lower than in the crystal phase. In the nematic phase, for example, the particles self-organize by all aligning along a common direction (the nematic director), while keeping their centers of mass homogeneously distributed in space. Chiral nematic liquid crystals are peculiar as their nematic director rotates like a helix around a chiral director, thus giving rise to a chiral distribution of the orientations of the particles [1]. A cholesteric phase can be either righthanded (as in Fig. 1(a)) or left-handed, depending on the handedness of the helix drawn by the nematic directorn. The wavelength associated to a full rotation of the nematic director around the chiral directorχ is known as the cholesteric pitch P . Cholesteric phases are commonly found in both thermotropic molecular compounds (e.g., derivatives of cholesterol [2][3][4][5]) and in lyotropic colloidal suspensions of, e.g., DNA [6,7] and filamentous viruses [8][9][10][11][12]. Their widespread occurrence explains why cholesterics were the first liquid-crystal phase experimentally observed [2]. The pitch is experimentally known to take values several orders of magnitude higher than the size of the constituent particles, reaching the visible-light wavelength in molecular compounds. For this reason, and for their liquid-like rheological properties, cholesterics have long found wide technological application in the opto-electronic industry [13].Despite their long history and widespread technological applications, surprisingly little is understood about this chiral state of matter. A fundamental open question regards the relationship between macroscopic and microscopic chirality, i.e., between the handedness of the phase and that of the constituent particles [14][15][16][17]. Moreover, it is still to be unambiguously proved that hard chiral interactions alone can give rise to an entropy-stabilized chiral-nematic phase [17,18]. The problem in interpreting experimental data is largely due to the limitations of theory and simulation method...

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confidence: 99%

“…Here we focus on the chiral nematic phase developed by a large class of hard-sphere helices described in Ref. [18] (cf. Fig.…”

mentioning

confidence: 99%

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confidence: 99%

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Density functional theory for chiral nematic liquid crystals

et al. 2014

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“…1(a)]. Hence different helix morphologies can be achieved upon changing r and p, 8 as in the experiments on flagella. 5 We then performed MC isobaric-isothermal (NPT) numerical simulations, 9 on systems of N, typically between 900 and 2000, such helices, at many values of pressure P, measured in reduced units P * = PD 3 /k B T, k B being the Boltzmann constant and T the temperature.…”

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confidence: 95%

“…As pitch increases, the location of the I-N phase transition moves to lower η, as indicated by the P 2 behavior, in agreement with results reported earlier. 8 This can be understood in terms of an increase of the effective aspect ratio that tends to stabilize the N phase. The location of the N-N * s phase transition instead moves to larger η for increasing pitch, with a significant pitch dependence, and with the N * s phase always occurring at high values of P 2 .…”

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Hyperspectral imaging with in-line interferometric femtosecond stimulated Raman scattering spectroscopy

Dobner

Fallnich

2014

The Journal of Chemical Physics

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We present the hyperspectral imaging capabilities of in-line interferometric femtosecond stimulated Raman scattering. The beneficial features of this method, namely, the improved signal-to-background ratio compared to other applicable broadband stimulated Raman scattering methods and the simple experimental implementation, allow for a rather fast acquisition of three-dimensional raster-scanned hyperspectral data-sets, which is shown for PMMA beads and a lipid droplet in water as a demonstration. A subsequent application of a principle component analysis displays the chemical selectivity of the method.

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Theory and Simulation Studies of Self‐Assembly of Helical Particles

Cinacchi

Ferrarini

Frezza

et al. 2016

Self‐Assembling Systems

Self Cite

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While it is possible to control the enantioselective process by using depletion interactions and faceting of the bulding blocks [5], helices are among the natural objects to focus on when dealing with chirality. New functional materials [9] [10] can be produced by exploiting the intrinsic chirality of the helical structures, which are useful in catalysis and demixing of enantiomers [11] [12]. The importance of the helix in nature is unquestionable: proteins, polysaccharides, DNA and RNA, the so called molecules of life, have a helical structure. The helical shape is exhibited in nature also by microorganisms, like filamentous viruses, and cell organelles, like bacterial flagella. Filamentous viruses are formed by a DNA of RNA core, wrapped by a coating of helically arranged proteins. Well-known examples are Tobacco Mosaic Virus (TMV), the first discovered virus [13], and viruses related to filamentous phage fd, whose mutants are present in nature (M13, fd), while others can be obtained by genetic engineering. They have been widely investigated as models of highly anisotropic, colloidal systems, with the advantage of being essentially monodisperse and that their length, of the order of a micrometer, makes them suitable for imaging techniques, such as optical microscopy. Bacterial flagella are helical macromolecular structures assembled from a single protein (flagellin). Their helical shape can be tuned with high precision by regulating external parameters such as temperature or pH, and their large size, of the order of microns, makes them very handy for optical observations. Because of their shape anisotropy, helical biopolymers and colloidal particles may exhibit liquid crystal phases at high densities [14]. These phases are often tacitly assumed to be the same as those occurring in systems of rod-like particles. However, it cannot be taken for granted that at such high densities the intrinsic helicity of the shape can be neglected. To explore the effect of self-assembly of helical polymers and colloids, and in particular to discover whether there is anything special just determined by the helical shape, we have undertaken a comprehensive investigation of the phase behavior of hard helices [15][16][17][18][19], interacting through purely steric repulsions, using Monte Carlo simulations and an extension of Onsager theory [20], a density functional theory (DFT) that was originally proposed to explain the onset if nematic ordering in a system of hard rods. These studies have revealed an unexpectedly rich phase behavior, the most interesting result being the existence of special phases characterized by screw -like ordering. Such kind of organization had been proposed for DNA, based on theoretical considerations [21], and had been observed in dense suspensions of flagellar filaments [22]. Hard helices are an athermal system: phase transitions are controlled by density and are driven by the entropy gain on moving from the less to the more ordered phase. This is a minimalist model, possibly insufficient to account entir...

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The isotropic-to-nematic phase transition in hard helices: Theory and simulation

Cited by 32 publications

References 53 publications

Density functional theory for chiral nematic liquid crystals

Density functional theory for chiral nematic liquid crystals

Hyperspectral imaging with in-line interferometric femtosecond stimulated Raman scattering spectroscopy

Theory and Simulation Studies of Self‐Assembly of Helical Particles

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